The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera equipped in a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called smart phones have been more widely used. The smart phones are cellular phones with functions of portable information terminals (PDA) and/or personal computers. Since the smart phones generally have more functions than those of the cellular phones, it is possible to use images taken by a camera thereof in various applications. For example, while it is possible to use the smart phones for printing and enjoying images taken, it is also possible to use images in other usage such as processing images to use for game characters or for makeup simulations, dress fitting simulations, and the others. The ways of the image usage were not conventionally common, however, it becomes more common mainly among young people.
In case of the smart phones, products are often available with various specifications ranging from the ones for beginners to the ones for advanced users. Generally, for an imaging lens for mounting in a cellular phone or a smart phone, which is developed for advanced users, it is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element of these days. On the other hand, in case of an imaging lens mounting on a smart phone for the above-described usage, it is rather important to be a smaller size and a wider angle of view, namely a wide angle, than to be a high resolution. Especially in these days, there are demands for an imaging lens to be a smaller size and a wider angle than conventional ones, associating with the fact that the smart phones become smaller and have more functions.
A lens configuration composed of four lenses is considered to be as one of the most suitable lens configurations in order to solve the above-described problems. As for an imaging lens configuration composed of four lenses, Patent Reference 1 and Patent Reference 2 have been known. The imaging lenses described in Patent References include a first lens that is negative and is a meniscus lens directing a concave surface thereof to the object side; a second lens that is a biconvex lens; a third lens that is negative and is a meniscus lens directing a concave surface thereof to the object side; and a fourth lens that is positive.
According to the configuration described above, the first lens is formed as a meniscus lens directing a concave surface thereof to the object side, thereby making it possible to achieve the wider angle of the imaging lens. In addition, the third lens is formed as a meniscus lens directing a concave surface thereof to the object side, so that it is possible to keep an angle between an incident light beam and an emitting light beam (angle of deviation) small and to restrain generation of aberrations on each reflecting surface.    Patent Reference 1: Japanese Patent Application Publication No. 2004-361934    Patent Reference 2: Japanese Patent Application Publication No. 2005-31638
According to the imaging lenses described in Patent Reference 1 and Patent Reference 2, it is possible to reduce a size of the imaging lens while securing the relatively wide angle of view. However, attaining more functions and the size reduction have been proceeding each year, and consequently, demand level for even wider angle and smaller size for the imaging lenses are higher than before. With the lens configuration described in Patent Reference 1 or Patent Reference 2, there is a limit by itself for attaining both the wide angle and the small size to satisfy the demands.
Further, the problem is not specific only to the imaging lens for mounting in small-sized portable devices such as cellular phones and smart phones, but also it is a problem for an imaging lens for mounting in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of solving the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can attain the wider angle and the size reduction.
Further objects and advantages of the invention will be apparent from the following description of the invention.